Reflected wave navigation device



. May 6, 1947. n. c. SANDERS. JR l 2,420,017

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forni Mmmm/- A u calage* May 6, 1947.` R. c. SANDERS, JR 2,420,917

REFLECTED WAVE NAVIGATION DEVICE Filed May 25, i944 2 Sheets-Sheet 2 tented REFLECTED WAVE NAVIGATION DEVICE Royden C. Sanders, Jr., Hightstown, N. J., alssignor to Radio Corporation of America, a corporation of Delaware Application May 23, 1944, Serial No. 537,021

9 Claims. l

This invention relates to aircraft navigation and more particularly to improvements inthe art of controlling automatically the direction of travel of an aircraft or other mobile craft to obtain approximately linear motion toward a selected target or other objective.

. It ig well known 'in prior practice to direct automatically the travel of a mobile .craft by means of a sighting device or radio direction iinder which is maintained in alignment with the objective. Ii the direction of travel is continuously maintained directly toward the objective, a so-called homing course is followed. In the presence of drift caused by cross wind, or in case the objective is a target moving with a component transverse to the line of sight, the course will be a curved path. The term line of sight as used herein is intended to mean the line between the objective and the craft as determined in any manner, for example by means of radio direction finder apparatus.

A so-called navigation course, or linear path of travel may be fllowed by maintaining the direction of motion of the craft at an angle with respect to the line of sight such that the I transverse component caused by target motion or drift is compensated by an equal transverse component in the direction of night of the craft being controlled. The necessary adjustment of direction of travel may be made readily if the magnitude of the transverse velocity component is known or can be calculated from available information. However, such information is frequently not available, particularly in the case of a moving target. As explained more fully hereinafter, a navigation course may be obtained without the necessity of knowing the transverse velocity, by maintaining a course such that the angle between the course and the line of sight remains constant. This may be accomplished by steering the craft in response to the sighting means s'o that as the line of sight changes through some angle Aa, the direction of travel changes through an angle una, where n is greater than l. The factor n is known as the navigation ratio. The greater the navigation "atio, the more nearly a true navigation course is approached; the less the navigation ratio, the more nearly a homing course. Owing to various practical considerations, the value of n is usually limited to a range of 1%/2 to 6, depending upon the characteristics of the particular system. Ordinarily the navigation ratio n is maintained constant.

The principal object of the instant invention is (Cl. Z50-1.54)

to provide an improved method of and means for navigationin which the aforementioned navigation ratio iscontrolled as a function of the predicted time which will be required to reach the objective. Another object is to provide a method of and means for controlling said navigation ratio automatically as a function of said time from objective. These and other objects will become apparent to those skilled in theart upon consideration of the following description, with reference to the accompanying drawings, of which:

Figure 1 is a geometrical diagram illustrating a. typical homing course, an ideal navigation course, and drift corrected courses, and Figure 2 is a schematic diagram of an aircraft control system embodying the invention.

Referring to Figure 1, assume that a target, starting at a point A, moves with a speed v along the course AC. Successive positions of the target at the ends of successive equal time intervals are indicated by the points 1, 2, 3, etc. A target seeking craft or seeker starts at the point B, moving with a speed V. In order for the seeker to intercept the target in the shortest possible time, the motion of the seeker must include a component Vd, perpendicular to the initial line of sight AB, which is equal lto the component vd of the motion of the target perpendicular to the line AB. To meet this condition, the motion of the seeker is directed at an angle 1E a-sm V with respect tothe line of sight BA. Successive positions of the seeker along this ideal navigation course at the ends of successive equal time intervals are indicated by the points 1', 2', 3', etc. The intervals are equal to those corresponding' to the respectively numbered points 1, 2, 3, etc. of the target course. It is to be noted that the angle a between the line Oi' Sight and the seeker course remains constant up to the point of interception, which occurs shortly after the end of the tenth interval.

Thus if the transverse velocity vd ofthe target with respect to the initial line of sight were known, and constant, there would be no dimculty in setting the ideal straight line interception course for the seeker. In practice, however, the

velocity vd is not usually known, or readily determinable; it is unlikely to remain constant, since a change in either the speed or the direc, tion 'of the target motion will alter the magni tude of vd. Furthermore, any drift of the seekerl relative to the ta'get must be added to or inillustrated by the lowest curve of Figure 1, Whereon successive positions of the seeker at the endsl 'oi successive equal time intervals are indicated by the-points 1", 2", 3,"etc. -The line ofsight, from the seeker at any point on the course to the target at the corresponding point on the course d Ierred that a large navigation ratio is to be desired, so as to approach the ideal navigation course as nearly as possible. This is not necessarily true however, for the following reasons: 1. There is a ilnite minimum change in sight angle which can be detected. 2. Response to .a

change in sight angle cannot be entirely instantaneous. but requires a iinite length of time. 3. Random variations in the apparent sight angle will occur for various reasons, particularly if radio direction iinding apparatus is employed for l "sighting" Assume that the minimum change in sight angle to which the seeker can respond is AC, is tangent to the curve, i. e., the angle a is v Zero.

As the seeker approaches the target, the course continuously increases in curvature, the seeker finally overtaking the target at approximately the end of the twelfth interval. under the condi-1 tions oi Figure 1. The ratio V/v o! seeker speed to target speed is approximately 2 in Figure 1. It4 the ratio were less. the seeker would follow a course similar to that shown, but with less curvature at each corresponding point on'the seeker course. The homing course thus has the theoretical disadvantage that the seeker can never reach the actual center of the target, and the much more practical disadvantage that the seeker course at the time of interception of the target is substantially curved. This places stringent speed of response requirements upon the seekers automatic control apparatus even ii the target hw quite appreciable size. and makes it impossible for the seeker to drop bombs accurately if the homing course is used.

'A course intermediate the homing course and the ideal navigation course may be obtained by `controlling the direction of travel of the seeker in response to continuous or repeated intermittent measurement of the angle a so that as a changes by an amount Aa, the seeker heading -is changed by an amount nAa in such direction as to oppose the change`in The courses represented by the curves designated F. G, and H in Figure 1^result from the use of values of 2, 3 and 5, respectively, for the navigation ratio n. In each case, the sight angle changes from its initial value of zero to a substantially constant finite value which is maintained to the point of interception. It is assumed that the initial value of a is zero both for simplicity in explanation and `because it is preferable in practice to start on a homing course, with a=0. to avoid a large initial correction which is not representative of the 'actual rate of change of sight angle and would require removal by correction in `the opposite direction.

With n='2, (curve F) the course is perceptibly curved throughout its entire length, since under the conditions of Figure 1. there is insufficient time for substantially complete correction to be one degree; this is a typical value for automatic radio direction iinder equipment. With a, naviy gation ratio oi 6, the smallest change in seeker course that can be made is 6 degrees. Suppose that the lag in the response of the steering mechanism is sufficient to allow a further change of sight angle during the time that correction is being made for the iirst one degree change. This further change will be greater than that which would have occurred if the correction had been made instantaneously, and may easily exceedv one degree. particularly if the seeker is near the target, causing a further correction of 6 degrees to be introduced when as a matter of fact, none is called for. The net eiect is thus that oi increasing the navigation ratio momentarily to 12. An abrupt change o! course may occur. causing a reversal of the direction of change of sight.

angle and, after a further period of delay, a large reverse correction of course. The seeker will y tend to follow an undulating path rather than a single-curved, gradually straightening course.

With large navigation ratios, random momentary variation of apparent sight angle, resulting 4 frommultipath radio transmission, for example, may initiate disturbances of the above described type. When radio echo type object locating and direction determining equipment is used, variationl in apparent sight angle becomes more troublesome as the target is approached because of the increase in angle that e. finite target will present and the shift in the apparent point of distances from the target.'strong reilections lfrom other objects near the line of sight may cause the seeker to switch over to an unsought or undesired target. With large navigation ratio, the false drift correction introduced by momentary change of target will be large,

The choice of navigation ratio is thus a compromise between rapidity of correction to a linear course, and stability. Heretofore, the practice has been to select a value which appears to give the best results, maintaining the ratio constant made. However. it is evident \that the curvature decreases as the seeker approaches the target, which is intercepted near the end of the eleventh interval. When n is increased to 3, (curve G). the seeker course is more sharply curved near the beginning, and becomes substantially linear after4 throughout the run. The advantage of a large navigation ratio is chiefly in shortness of the time necessary to establish proper drift course. See curve H. The use of a large navigation ratio makes it possible to delay the use of navigation until the seeker is relatively close to the target,

thus improving the selection of a desired target from a plurality of reilecting objects. After a linear path is obtained, reduction of the navigation ratio will not change the course. The disadvantage of a large navigation ratio is more apparent as the target isapproached. By decreasing lthenavigation ratio as the time from target decreases, the advantage of rapidly setting a navigation course is obtained, without the disadvantage of unstable operation near the target.

The system of Figure 2 is a practical embodiment oi the invention for controlling an aircraft iable capacitor type.

asseoir port 3 is connected to a vertical shaft il, which is connected through diierential gearingI i3 to a o stabilizer i3. The gyro i3 maintains its output shaft l1 at a constant angle in space, independently of motion of the craft upon which the equipment is mounted. The spider of the e; erential i3 is geared to a shaft I9. Rotation ot the shaft i3 thus rotates the antennas 3 and 1 to angular position in space corresponding to the position o3 the shaft i3.

d receiving antenna 2i is connected to a receiver 33 which includes a heterodyne detector. The receiver 23 is coupled to the transmitter i by suitable means, such as a transmission line The output circuit of the receiver 23 is connected to a rectifier 21 which includes a load resistor 23. The upper end of the resistor 29 is coupled through a capacitor 3i to a double throw switch 33. The lower end of the resistor 23 and the lower ilxed contact of the switch 33 are connected to ground. The upper fixed contact of the switch 33 is connected through a low pass lter 33 to a D.C. ampliner 31. The output circuit oi the D.C. amplier 31 includes a, relay 33.

The contacts of the relay 33 are connected be- I tween a battery l and a reversiblemotor 33 in such manner that when the current through the winding oi the relay 33 is above a predetermined value, the motor 33 runs in one direction, and when the current is below the predetermined value. the motor 33 runs in the other direction. Preferably, said values differ somewhat so as to provide a "dead space, or range of current of intermediate value wherein the motor 33 is deenergized. 'Ihe motor 33 is coupled to the input shalt i3 oi the diderential 13.

The switches 3 and 33 are operated in synchronism by means of cams 33 and 31 on the shalt 33 ci a motor 3l. The motor 3i is'ener-v gleed by a battery 33 connected thereto through a switch 33. The cams 33 and 31 are so positioned that the transmitter Il is connected to the antenna 3 during the time that the switchv varied in frequency by means of a frequency modulator 31, which may be oi the vibratory vargized by impulses produced by connecting periodically a battery 33 to a resistor 31 through a switch 33. The switch B3 is arranged to be operated by a cam 33 on the shaft 33. The cam 33 includes two lobes, so that the switch i is operated through a complete cycle during each period oi connection of the transmitter i to each of the antennas 3 and 1. The resulting square wave `volte which appears across the resistor '3l is converted to triangular wave form byy means oi a, wave shaping circuit 33, which may be an integrating circuit of the low pass illter type.

The operation of the system as thus far demrlbed is as follows: The switch 3 is cycllcally driven irom one position to the other, connecting the transmitter l alternately to th'e antennas 3 and 1. Signals are thus radiated alternately in two overlapping directive patterns. a .reecting The modulator 31 is ener-l of antennas 3 and 1 will return a signal to the receiving antenna 21.. This signal is combined in the heterodyne detector of the receiver 23 with a signal transmitted directly through the line 23 from the transmitter I. Owing to the delay in the reflected signal with' respect to the directly transmitted signal, the two inputs to the receiver 23 differ in instantaneous frequency by an average amount proportional to the distance of the reiiecting object from the antennas. When the distance is decreasing due to motion of the seeker craft toward the target, the diierence is larger when the transmitted frequency is decreasing, and smaller when the transmitted frequency is increasing, as a result of the Doppler increase in y frequency of the received signal. The receiver 23 3 is operated. If the reflecting object lies to the left oi' the equi-signal line, for example, a stronger signal will be returned to the antenna 2l when the switch 3 is in its lower position, and the beat signal will fluctuate in amplitude between two values which differ in accordance with the azimuth of the reecting object with respect to the equi-signal line.

the variations in amplitude of the beat voltage.

When the switch 33 is in its lower position, the capacitor 3i is charged to a voltage corresponding to the strength of the signal reected from the antenna 1. When the switch 33 is in its upper position, the voltage across the resistor 29 corresponds to the strength of the signal reected from the antenna 3. This latter voltage is opposed to that across the capacitor 3i, so that the net voltage applied to the lter 33 is proportional to the difference between the strengths of th'e two reected signals. This dierence voltage is smoothed by the filter 33 and applied to the D.C. amplilier 31. The amplifier 31 is biased so that when the input thereto is of one polarity, the relay 3B is actuated to its upper position, and when the input is oi' the other polarity, the relay 39 drops to its lower position. I

Accordingly, th'e motor d3 is energized to rotate the shaft i9 and hence the shaft Il, turning the antennas 3 and 1 to a position such that their signals are reected with equal strengths to the antenn'a, 2l. Thereafter, any change in position of the reflecting object with respect to the equisignal line will be compensated automatically, by operation of th'e motor 33 so as to maintain the equi-signal line directed toward the object.

Initial selection of the object upon which the bearing is to be maintained is accomplished by centering the antennas with respect to the longitudinal axis of the seeker craft, and steering manually toward the desired object or target to pick up a. reilection from it. An antenna centerk ing switch 31 is provided with a movable contact 1 33 coupled to the shaft Il and two iixed contact motor 15 and a power source 11, through a manually operable switch 19. The motor 15 is coupled to the gyroscope I5 in such manner that as the motor 15 rotates, the gyro is caused to precess, rotating the shaft I1. Assuming that the shaft IS is held in a xed position, closure of the switch 19 will cause the motor 15 to precess the gyro I5, rotating the shaft II to center the antennas 5 and 1.

The antenna shaft I I is coupled to the steering mechanism by means of a servo system comprising a bridge circuit including two variable voltage dividers 8| and 83. The variable taps of the voltage dividers 8| and 83 constitute one pair of conjugate terminals of the bridge and are connected to a polarized relay 85. The contacts of the relay 85 are connected to control the energization ofa reversible motor 81 from a battery 89. The shaft of the motor 81 is connected to the steering mechanism (not shown). Corresponding terminals of the voltage dividers 8| and 83 are connected together through switches 9| and 93, arranged to include selectively fixed resistors 95 and 91 or variable resistors 99 and |0I in the connections. A battery |03 is connected across the voltage divider 83. The variable resistors 99 and |I are provided with a common shaft |05 which is coupled through a magnetic clutch |01 to a shaft |09. The clutch |01 is connected to a battery ||I through a switch II3. The shaft |09 is coupled through a variable ratio drive mechanism I| to the shaft I9.` The drive ratio between the shafts I9 and |09 is determined by the position of the control shaft I1, which is coupled to the reversible motor |29. Thus the position of the shaft of the motor |29 controls the ratio of the mechanism ||5.

A centering switch |3I, similar in construction to the switch 61, is coupled to the shaft |05 and connected to a two position switch |33. The switch |33 is connected to the contacts of the relay 39 and to the motor 03. When the switch |33 is thrown to one position, the motor ||3 is subject to the control of the relay 39. When the switch |33 ls thrown to its other position, the motor 03 rotates the shaft |05 to center the Variable resistors 99 and IOI, and maintain the shaft I9 in a fixed position while the antennas 5 and 1 are being centered.

The motor |29 is arranged to operate in accordance with the predicted time to reach the reflecting object as follows:

The output of the receiver 23 is applied through a limiter I 35 to a pair of averaging cycle counters |31 and |39. operated by the cam 66 is connected to the counters |31 and |39 to cause alternate operation thereof in synchronism with the operation of the switch 63. The counter |31 operates while the switch 63 is closed, and the transmitted frequency is increasing, and the counter |39 operates during the periods when the transmitted frequency is decreasing. Electronic switching means of the type disclosed in copending application Ser. No. 524,794, filed by R. C. Sanders, Jr., and- W. R. Mercer on March 2, 1944, and entitled Radio bomb release system may be substituted for the switch is desired. The counters |31 and |39 are provided with a common load resistor |00.

The positive output terminal of the counter |31 and the negative output terminal of the counter |31 are connected to the upper end of the resistor |40, which is connected to the input terminals of a D.C. amplifier |03. 'I'he return connections for the counter output circuits are connected scp- A switch IM, arranged to be beat frequency and the counter |39 provides a larger output in accordance with the velocity. The average current through the resistor |40 is proportional to the difference of the outputs of the two counters. If the counter sensitivities were equal, the voltage across the resistor |40 would be proportional only to speed. However, the counter |31 is designed to have a greater sensitivity (volts per beat frequency cycle per second) than the counter |39. Thus the average voltage drop in the resistor |00 is proportional to where k is the ratio of the sensitivity of the counter |31 to that of the counter |39, ,fu is the beat frequency during increase of the transmitted frequency and fa is the beat frequency during de.- crease of the transmitted frequency.

The output of the amplifier I 43 is connected to a relay |45 similar to the relay 39. The relay |45 is arranged to control the energization of the motor |29 from a battery |41. The shaft of the motor |29 is coupled to the voltage divider 6| which controls the amplitude of the square wave modulation signal. Thus the position of the shaft of the motor |29 controls the width.of the band through which the frequency of the transmitter I is varied.

The beat frequency during increase of transmitted frequency is f=S2f;'1-2gc cycles per second The average voltage across the counter load resistor |00 is proportional to Ykfffi= (1c-n? It is apparent that this voltage may be made zero by proper adjustment of the voltage divider 6I, which controls the band width S. The motor |29 is controlled by the relay |45 in response to the output of the amplifier |03 to adjust the voltage divider 6I for this condition. Thus 'The voltage divider ti is constructed with a reciprmal taper, that is the ratio of the output to the input over the operative portion of its range of adjustment is inversely proportional to the angular position of its control shaft. Thus, the position of the shaft of the motor |29 is continuously maintained at an angular position' which is directly proportional to the time from target T.

The adjustment and operation of the system for homing is as follows:

The switches 9i and 93 are thrown to their ,upper positions, connecting the fixed resistors 95 and 9i in the bridge circuit. The switches 'i9 and H3 are closed and the switch |33 is operated to its lower position, connecting the centering switch iti in the circuit of the motor 43.v The motors 'lt and t3 then operate to center the antennas E and 'i with respect to the longitudinal axis of the seeker craft. Ii' the angular positions of the movable arms of the voltage dividers 8| and t3 do not coincide, the differential relay 85 is operated, energizing the motor 8l to drive the voltage divider 83 and the steering mechanism to their center positions. The craft is then steered manuallytoward the desired object, by superimposing manual control of the steering mechanism upon that provided by the motor 31. The switches it and ||3 are then opened and the switch |33 is operated to connect the motor di to the relayr tu. As long as the target remains upon the equi-signal line of the antennas and 1. the motor t3 is deenergized. However, if` the line of sight to the target changes, the motor it is energized accordingly, rotating the shaft i9 and thereby turning the shaft 'il to again direct the antennas toward the target. as described above.

Rotation of the shaft il turns the voltage divider di, unbaiancing the bridge circuit and operating the relay 8b, thus energizing the motor B1 to that of the shaft fi.

, same direction as the line of sight correction,

lIl() causing the motor 81 to rotate further than the shaft Ii by an amount depending upon the rotation of the shaft |05. The ratio of the angle through which the shaftof the motor 8l lrotates to that through which the shaft rotates is the navigation ratio, and is controlled by the motor |28. The proportionality of the navigation ratio to the time T may be controlled by adjustment ofthe voltage of the battery |41 or by means of change gearing, not shown, interposed between the drive i5 and the shaft |08.

At the beginning of the' run. when the seeker craft is relatively far from the target, after a brief period of homing, the motor |28 will be at a position near the clockwise limit of its motion, providing a high navigation ratio. As the distance to the target decreases, the motor |28 reduces the drive ratio. providing a correspondingiy decreasing navigation ratio.`

The seeker will tend to attain a relatively linear course similar to the curve H of Figure l. The subsequent reduction of the navigation ratio y will minimize the tendency to deviate from this course as the Vtarget is approached.

Although the inventionhas been described with reference to a radio locator system ofthe .El-M. reectiontype, it will be evident that it may be embodied in any system wherein a mobile craft is steered in response to the line of sight of the objective, by controlling the navigation ratio as a function of the distance.

I claim as my invention:

1. The method of steering a mobile craft toward a predetermined objective. including the steps of substantially continuously determining the azimuth of said objective with respect to the line of travel of said craft, changing the direction' of travel of said craft in response to changes occurring in said azimuth but to extents which are multiples of said changes'in' azimuth by a factor designated as the navigation ratio, subcontrols the steering mechanism. altering the course of the craft to cause it to again travel toward the target.

To provide an interception or navigation course, the equipment is first operated as dethe variable resistors 98 and IM for the resistors it and t? in the bridge circuit. The switch Iii is simultaneously closed to engage the clutch Il.

, change in the bearing of the target, the motor di operates to restore the alignment of the antennas tothciineofeisht. Atthesametimeasthisad-A scribed above for homing. After a brief period of homing omration, the switches 9| and 93 are operated to their lower positions, substituting` jective, and varying the value stantially continuously determining the predicted time required for said craft to arrive at said obof said navigation ratio directly as said predicted time.

2. The method of steering a mobile craft toward a predetermined objective, including the steps of varying the direction of travel of said craft in response to variation in the azimuth of said object with respect to the line of travel of said craft, but to an extent which is greater than said variation in azimuth by a. factor designated as the navigation ratio, substantially continuously determining the predicted time required for said craft V.to reach said objective, and varying the value of said navigation ratio as a predetermined function of said predicted time.

' 3. A target seeking system for mobile craft. including target azimuth responsive steering means, variable ratio drive mechanism connected to said azimuth responsive means to control the extent of the response thereto to a given change of azimuth, and means responsive to the quotient of target distance and rate of change of target from said transmitter ations by a navigation ratio, the system for con-` trolling the value of said navigation ratio includ ing a time from target predicting device, a. variable ratio drive system interposed in said steering system, and means for controlling the ratio of said variable ratio drive system in response to said time predicting device. Y

5. A target azimuth responsive steering system for mobile craft, including means for transmitting a frequency modulated signal toward a selected target, means for receiving said signal after reflection by said target and for combining said reflected signal with a signal received directly to provide a beat signal having an average frequency which bears a predetermined relationship to the distance of said target from said craft, and variation which bears a predetermined relationship to the speed of said craft with respect to said target, said transmitter means and said receiver means each lincluding an antenna, means for cyclically altering the directivity of at least one of said antennas to provide two overlapping 'directive patterns intersecting in an equi-signal line, whereby said beat signal is variedin amplitude in accordance with deviation of said equisignal line from the azimuth of said target, means for establishing a reference line of fixed azimuth in space. means responsive to said variations in amplitude of said beat signal for varying the angular position of said antenna whose directivity is cyclically altered to vary correspondingly the angular position of said equi-signal line with respect to said reference line, whereby said vequisignal line is maintained in a direction corresponding to said target azimuth, a steering mom ton means responsive to variation in the posi tion of said equi-signal line with respect to the position ci said reference line to vary in a corI responding direction the angular positionof the shaft of said steering motor through an angle which is related to the angle of variation of said equi-signal line by a factor designated as the navigation ratio, and means responsive to said beat signal to control the magnitude of said navigation ratio as a predetermined function of the predicted time required for said craft to arrive at said target.

A target azimuth responsive steering system for mobile craft, including means for transmitting a frequency modulated signalV toward a selected target, means for receiving said signal after reflection by said target and for combining said reflected signal with a signal received directly from said transmitter to provide a beat signal having an average frequency which bears a predetermined relationship to the distance of said target from said craft, and a range of frequency variation which bears a predetermined relationship to the speed of said craft with respect to said target, said transmitter means and said receiver means each including an antenna, means for cyclically altering the directivity of at least one of said antennas to provide two overlapping directive pat terne intersecting an equi-signal line, whereby said beat signal is varied in amplitude in accordance with deviation of said equi-signal line from the azimuth of said target, means for establishing a reference line of fixed azimuth in space, an antenna positioning motor coupled to said an-a tenna for varying theangular position of said an tenna to vary correspondingly the angular position of said equi-signal line with respect to said V reference line, control means for said motor rea range of frequency sponsive to said variations in amplitude oi said beat signal to control the angular position of said antenna whose directivity is cyclically altered to maintain said equi-signal line in a direction corresponding to said target azimuth, a steering motor, control means for said steering motor responsive to variation in the position of said antenna positioning motor to vary in a corresponding direction the angular position of the shaft of said steering motor through an angle which is related to the angle of variation of said equi-signal line by a factor designated as the navigation ratio, and means responsive to the frequency of said beat signal to control the magnitude of said navigation ratio as a predetermined ratio of the predicted time required for said craft to arrive at said target.

7. A target azimuth responsive steering system for mobile craft, including means for transmitting a frequency modulated signal toward a selected target, means for receiving said signal after reflection by said target and for combining said reiiected signal with a signal received directly from said transmitter to provide a beat signal having an average frequency which bears a predetermined relationship to the distance or said target from said craft, and a range of frequency variation which bears a predetermined relationship to the speed of said craft with respect to said target, said transmitter means and said receiver means each including an antenna, means for cyclically altering the directivity of at least 'one of said antennas to provide alternately two overlapping directive patterns intersecting an equi-signal line, whereby said beat signal is varied in amplitude in accordance with deviation of said equisignal line from the azimuth of said target, gyroscope means for establishing a reference line of fixed azimuth in space, an antenna positioning motor for varying the angular position of said antenna whose directivity is cyclically altered to vary correspondingly the angular position of said equi-signal line with respect to said reference line, control means for said antenna positioning motor responsive to said variations in amplitude of said beat signal to control the angular position of said antenna to maintain said equi-signal line in a direction corresponding to said target azimuth, a steering motor, control means for said steering motor responsive to variation in the position of said antenna positioning motor to vary in a corresponding direction the angular position of the shaft of said steering motor, and means respon sive to the frequency of said beat signal to control the extent of the response of said steering motor control means to said variation in position of said antenna positioning motor as a predetermined function of the ratio of said average frequency of said beat signal to said range of frequency variation of said beat signal.

8. A. target azimuth responsive steering system for mobile crait including a time from target predicting system, a target azimuth determining sysn tem. a servo system responsive to said azimuth de- "termining system to steer said craft, and means responsive to said time from target predicting system to control the extent of the response oi.' said servo system to said azimuth determining system.

9. A target azimuth responsive steering system for mobile craft including a time from target predicting system of the radio reflection type. said system including directional antenna means and servo means responsive to the amplitude of signais reiiected from a selected target to orient said antenna means in a predetermined angular rela- RQYDEN C. S i

me, JR.

mmm?

The following references are ofrecord in the le of this patent: f'

A mf STATES 2 Number Name Date Moueix Oct. 17, 1939 v 

